Pyridazin-3-one derivatives of formula (7): ##STR4## wherein R.sup.2 and R.sup.3 are independently hydrogen or C.sub.1 -C.sub.3 alkyl, and Q is optionally substituted phenyl, have excellent herbicidal activity, including the following examples: ##STR5## The production of pyridazin-3-one derivatives in a favorable manner is preferred for the development of pyridazine herbicides with excellent activity.